An active tie rod has initially the function of retaining in a stowed configuration one or more spacecraft appendages, i.e., generally speaking, mobile elements deployable on either side of the body of the satellite, and after that releasing those appendages without shock in order to deploy them. The system for retaining the appendages in the stowed configuration and releasing them conventionally consists primarily of a separation nut and a tensioning tie rod. In concrete terms, in known devices, the tie rod participates with the nut in tensioning a series of stacked elements such as solar generators folded on one another with the aim of minimizing the volume of the satellite when it is disposed in the nose cap of a launch vehicle.
However, it is known that in this type of device the tie rod can have large geometrical defects, in particular if the number of stacked elements is large. The geometrical defects may be defects of inclination or of delocalization, for example. These geometrical defects can generate dynamic defects reflected in radial forces or moments exerted at the ends of the tie rod.
Failure to take account of these defects can generate serious malfunctions, up to non-deployment of the spacecraft appendages, and therefore possibly implying failure of the mission.
Another known problem lies in the shock that may be produced on releasing the tie rod. The aforementioned nut is responsible for releasing the tie rod. Now, at the moment of its release, the tie rod is in a stressed state and its sudden release can cause shocks that can degrade the spacecraft appendages or the satellite itself. There is a two-fold problem in the context of some present-day devices. In fact, the tie rod is initially released by a pyrotechnic latch the explosion of which generates a first shock. Thereafter, the tie rod being under tension before it is released, its sudden release, already referred to, can cause deterioration at the level of the spacecraft appendages or the supporting structure.
With the aim of minimizing this problem, so-called low shock nuts have been developed. This type of nut, which is complex, is generally of the single-use type and can be reconditioned. However the “low shock” capability of these nuts is applicable only to triggering the release of the tie rod and not the release of the appendages themselves. In fact, the geometrical defects of the tie rod are not really compensated and, in particular if the tie rod is of large size, there continues to be a sudden relaxation of the tie rod that can generate a shock that is sometimes intense, the tie rod striking the spaced appendages. This is notably explained by the fact that the release of the stressed tie rod continues to be sensitive to the angular deviations of the tie rod. The applicant has even noted that the shocks generated by the relaxation of the tie rod are most often much more severe than those generated by the nut, even if it is a pyrotechnic nut.
The patent EP 2 319 764 B1 filed 1 Oct. 2010 in the name of the applicant describes a tie rod made from a material having controlled thermal expansion. By heating the tie rod it is possible to lengthen it before releasing the nut and therefore to limit the shocks caused by the sudden relaxation of the tie rod. This solution has the disadvantage of limiting the choice of the materials from which the tie rod can be made. In fact, the required thermal expansion may be incompatible with the mechanical strength necessary to fulfil the function of retaining the appendages, notably during the launch phase in which the satellite is subjected to high levels of vibration. Moreover, the tie rod may be very long and it is necessary to heat it over its entire length to expand it. The implementation of the heater over the entire length of the tie rod is a complicated process.